The invention relates to organic light emitting devices, and more particularly, to enhancing optoelectronic performance by plasma treatment of conductive layers utilized in organic light emitting devices.
Organic light emitting devices (OLEDs) employ organic thin film materials which emit light when excited by electric current. These devices typically consist of a sandwich structure with organic thin films deposited onto a transparent substrate and covered by top metal cathode contacts. A layer of transparent and conductive material is interposed between the transparent substrate and the organic thin films to serve as an anode.
The organic thin films typically consist of an emission layer between an electron transport layer and a hole transport layer. When current is applied between the cathode and anode, the emission layer provides a recombination region for electrons injected from the electron transport layer and holes from the hole transport layer. This recombination results in the emission of light having a characteristic wavelength dependent on the organic materials used. Alternatively, single-layer organic or blended organic layers can be used instead of multilayer organic thin films.
Due to its transparency, high conductivity and work function as a hole injector into organic materials, indium-tin-oxide (ITO) is widely used as the anode material for OLEDs. Since the hole transport layer is in direct contact with the ITO in OLEDs, the surface work function of the ITO is expected to approach that of the hole transport layer, enhancing device performance. Consequently, a plasma treatment is performed to modify the surface work function of the ITO prior to formation of the hole transport layer thereon.
U.S. Pat. No. 6,259,202 to Sturm et al., the entirety of which is hereby incorporated by reference, discloses a method for modifying a work function of an ITO layer, comprising subjecting the ITO layer to an oxygen plasma treatment and a hydrogen plasma treatment. Oxygen plasma tends to increase ITO work function and hydrogen plasma tends to decrease ITO work function.
The conventional techniques, however, employ a dangerous hydrogen gas, which posed serious threat to worker safety. Additionally, the conventional techniques do not teach how to precisely match the surface work function of the ITO with that of the adjacent hole transport layer.